Silicon carbide semiconductor substrate

ABSTRACT

A silicon carbide semiconductor substrate includes an epitaxial layer. A difference of a donor concentration and an acceptor concentration of the epitaxial layer is within a range from 1×10 14 /cm 3  to 1×10 15 /cm 3 . Further, the donor concentration and the acceptor concentration of the epitaxial layer are a concentration unaffected by an impurity inside epitaxial growth equipment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.15/666,531, filed on Aug. 1, 2017, and is based upon and claims thebenefit of priority of the prior Japanese Patent Application No.2016-155091, filed on Aug. 5, 2016, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

Embodiments of the invention relate to a silicon carbide semiconductorsubstrate and a method of manufacturing a silicon carbide semiconductorsubstrate.

2. Description of the Related Art

Conventionally, single crystal silicon carbide (SiC) epitaxial(hereinafter, epitaxial may be abbreviated as “epi”) substrates, whichare single crystal SiC substrates on which a single crystal SiC isformed by epitaxial growth, have been researched, developed, and putinto practical use for applications such as high-voltage Schottky diodesof 1 kV and high-voltage metal oxide semiconductor field effecttransistors (MOSFETs).

However, to realize an ultrahigh-voltage, low-loss device of 10 kV, asingle crystal SiC epi substrate, which is a single crystal siliconcarbide semiconductor substrate on which a low-concentration singlecrystal SiC having an impurity concentration of 1×10¹⁴/cm³ to 1×10¹⁵/cm³is formed by epitaxial growth, has to be produced (manufactured).Hereinafter, single crystal SiC formed by epitaxial growth on a singlecrystal silicon carbide semiconductor substrate may be abbreviated as“epi deposited film”.

According to one technique, an epi layer of a low nitrogen (N₂)concentration of about 1×10¹⁴/cm³ to 1×10¹⁷/cm³ is formed as alow-concentration epi deposited film (for example, refer to JapaneseLaid-Open Patent Publication No. 2015-002207). According to anothertechnique, an epitaxial layer having an impurity concentration from1×10¹⁴/cm³ to 1×10¹⁶/cm³ is formed by epitaxial growth (for example,refer to Japanese Laid-Open Patent Publication No. 2012-253115).

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a silicon carbidesemiconductor substrate includes an epitaxial layer having a donorconcentration and an acceptor concentration that are equal to or higherthan a concentration unaffected by an impurity inside epitaxial growthequipment. A difference of the donor concentration and the acceptorconcentration ranges from 1×10¹⁴/cm³ to 1×10¹⁵/cm³.

In the silicon carbide semiconductor substrate according, a donor isnitrogen and an acceptor is boron.

In the silicon carbide semiconductor substrate, the concentrationunaffected by the impurity is 1×10¹⁵/cm³ or higher.

A method of manufacturing a silicon carbide semiconductor substrateincludes supplying a dopant gas including a donor and an acceptor, andforming a film of a single crystal silicon carbide on a single crystalsilicon carbide substrate by epitaxial growth. A flowrate of the donorand the acceptor in the dopant gas is equal to or higher than a flowrateby which a donor concentration of the film and an acceptor concentrationof the film are a concentration unaffected by an impurity insideepitaxial growth equipment. The flowrate of the donor and the acceptorin the dopant gas is a flowrate by which a difference of the donorconcentration of the film and the acceptor concentration of the filmranges from 1×10¹⁴/cm³ to 1×10¹⁵/cm³.

The method includes, before forming the film: supplying a dopant gasincluding the donor, forming a film of a single crystal silicon carbideon a single crystal silicon carbide substrate by epitaxial growth, andobtaining a first relationship of a flowrate in the dopant gas and adonor concentration of the film; supplying a dopant gas including theacceptor, forming a film of a single crystal silicon carbide on a singlecrystal silicon carbide substrate by epitaxial growth, and obtaining asecond relationship of a flowrate in the dopant gas and an acceptorconcentration of the film; and determining the flowrate of the dopantgas based on the first relationship and the second relationship;

In the method, the donor is nitrogen and the acceptor is boron.

In the method, the concentration unaffected by the impurity is1×10¹⁵/cm³ or higher.

Objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a silicon carbide semiconductorsubstrate according to an embodiment;

FIG. 2 is a flowchart of formation processes of the silicon carbidesemiconductor substrate according to the embodiment;

FIG. 3 is a graph indicating a relationship of impurity concentration ofdopant at the time of doping and impurity concentration of an epideposited film on the silicon carbide semiconductor substrate accordingto the embodiment;

FIG. 4 is a cross-sectional view of a structure of epitaxial growthequipment; and

FIG. 5 is graph of results obtained when conditions were set andformation was performed to determine concentration dependency in aconventional silicon carbide semiconductor substrate.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of a silicon carbide semiconductor substrate and a method ofmanufacturing a silicon carbide semiconductor substrate according to thepresent invention will be described in detail with reference to theaccompanying drawings. In the present description and accompanyingdrawings, layers and regions prefixed with n or p mean that majoritycarriers are electrons or holes. Additionally, + or − appended to n or pmeans that the impurity concentration is higher or lower, respectively,than layers and regions without + or −. Cases where symbols such as n'sand p's that include + or − are the same indicate that concentrationsare close and therefore, the concentrations are not necessarily equal.In the description of the embodiments below and the accompanyingdrawings, main portions that are identical will be given the samereference numerals and will not be repeatedly described. Further, in thepresent description, when Miller indices are described, “−” means a baradded to an index immediately after the “−”, and a negative index isexpressed by prefixing “−” to the index.

FIG. 1 is a cross-sectional view of the silicon carbide semiconductorsubstrate according to an embodiment. As depicted in FIG. 1, in thesilicon carbide semiconductor substrate according to the embodiment, ona first main surface (front surface), for example, (0001) face (Siface), of an n-type silicon carbide substrate (single crystal siliconcarbide substrate) 1, an n⁻-type epitaxial layer (epitaxial layer) 2 isdeposited.

The n-type silicon carbide substrate 1 is a silicon carbide singlecrystal substrate. The n⁻-type epitaxial layer 2 is, for example, alow-concentration n⁻-type drift layer having an impurity concentrationlower than that of the n-type silicon carbide substrate 1. The n⁻-typeepitaxial layer 2 is a film formed by epitaxial growth. In the n⁻-typeepitaxial layer 2, a difference of donor concentration and acceptorconcentration is in a range from 1×10¹⁴/cm³ to 1×10¹⁵/cm³. Therefore,the impurity concentration of the n⁻-type epitaxial layer 2 is1×10¹⁴/cm³ to 1×10¹⁵/cm³. Further, in the n⁻-type epitaxial layer 2, thedonor concentration and the acceptor concentration are equal to orhigher than a concentration not affected by impurities inside theepitaxial growth equipment 4.

Here, a donor of the n⁻-type epitaxial layer 2 is, for example,nitrogen. An acceptor of the n⁻-type epitaxial layer 2 is, for example,boron (B). Further, a concentration not affected by impurities insidethe epitaxial growth equipment 4 is, for example, 1×10¹⁵/cm³.

Next, the method of manufacturing a silicon carbide semiconductorsubstrate according to the embodiment will be described. FIG. 2 is aflowchart of formation processes of the silicon carbide semiconductorsubstrate according to the embodiment. Further, FIG. 3 is a graphindicating the relationship of the impurity concentration of dopant atthe time of doping and the impurity concentration of the epi depositedfilm on the silicon carbide semiconductor substrate according to theembodiment. In FIG. 3, the vertical axis indicates the concentration ofnitrogen or boron in the epi deposited film formed on the siliconcarbide semiconductor substrate. The concentration is indicated in unitsof /cm³. The horizontal axis indicates flowrate of nitrogen or boron inthe dopant gas. The flowrate is indicated in units of sccm or slm.

In the method of manufacturing a silicon carbide semiconductor substrateaccording to the embodiment, first, a first condition-setting andformation session is performed in which nitrogen is added (step S1). Forexample, with the temperature of a silicon carbide semiconductorsubstrate from 1580 to 1725 degrees C., using hydrogen (H₂) gas as acarrier gas, silane (SiH₄) gas and propane (C₃H₈) gas are concurrentlysupplied to the surface of the silicon carbide semiconductor substrate,and a dopant gas including nitrogen is supplied, forming an epitaxiallayer. Here, concurrent supply means that the SiH₄ gas and the C₃H₈ gasare supplied within a few seconds of each other and not necessarilyexactly at the same time. After the first condition-setting andformation session, the concentration of nitrogen in the formed epitaxiallayer is measured. The nitrogen flowrate of the dopant and theconcentration of nitrogen in the formed epitaxial layer in the firstcondition-setting and formation session are plotted at point 31 in FIG.3.

Next, a second condition-setting and formation session is performed inwhich nitrogen is added (step S2). For example, with the temperature ofa silicon carbide semiconductor substrate from 1580 to 1725 degrees C.,using H₂ gas a carrier gas, SiH₄ gas and C₃H₈ gas are suppliedconcurrently to the surface of the silicon carbide semiconductorsubstrate, and a dopant gas including nitrogen is supplied, forming anepitaxial layer. After the second condition-setting and formationsession, the concentration of nitrogen in the formed epitaxial layer ismeasured. The nitrogen flowrate of the dopant gas and the concentrationof nitrogen in the formed epitaxial layer in the secondcondition-setting and formation session are plotted at point 32 in FIG.3.

Next, a relationship of the added nitrogen flowrate and theconcentration of nitrogen in the epi deposited film is calculated (stepS3). For example, the result point of the first condition-setting andformation session and the result point of the second condition-settingand formation session are connected by a straight line whereby therelationship of the added nitrogen flowrate and the concentration ofnitrogen in the epi deposited film is calculated. In FIG. 3, a line 33connecting point 31 and point 32 represents the relationship of theadded nitrogen flowrate and the concentration of nitrogen in the epideposited film.

Next, a third condition-setting and formation session is performed inwhich boron is added (step S4). For example, with the temperature of asilicon carbide semiconductor substrate from 1580 to 1725 degrees C.,using H₂ gas a carrier gas, SiH₄ gas and C₃H₈ gas are suppliedconcurrently to the surface of the silicon carbide semiconductorsubstrate, and a dopant gas including boron is supplied, forming anepitaxial layer. After the third condition-setting and formationsession, the concentration of boron in the formed epitaxial layer ismeasured. The boron flowrate of the dopant gas and the concentration ofboron in the formed epitaxial layer in the third condition-setting andformation session are plotted at point 35 in FIG. 3.

Next, a fourth condition-setting and formation session is performed inwhich boron is added (step S5). For example, with the temperature of asilicon carbide semiconductor substrate from 1580 to 1725 degrees C.,using H₂ gas a carrier gas, SiH₄ gas and C₃H₈ gas are suppliedconcurrently to the surface of the silicon carbide semiconductorsubstrate, and a dopant gas including boron is supplied, forming anepitaxial layer. After the fourth condition-setting and formationsession, the concentration of boron in the formed epitaxial layer ismeasured. The boron flowrate of the dopant gas and the concentration ofboron in the formed epitaxial layer in the third condition-setting andformation session are plotted at point 36 in FIG. 3.

Next, a relationship of the added boron flowrate and the concentrationof boron in the epi deposited film is calculated (step S6). For example,the result point of the third condition-setting and formation sessionand the result point of the fourth condition-setting and formationsession are connected by a straight line whereby the relationship of theadded boron flowrate and the concentration of boron in the epi depositedfilm are calculated. In FIG. 3, a line 37 connecting point 35 and point36 represents the relationship of the added boron flowrate and theconcentration of boron in the epi deposited film.

Here, the sequence of steps S1 to S3 and steps S4 to S6 may be reversed.In other words, after steps S4 to S6 are executed, steps S1 to S3 may beexecuted.

Next, the nitrogen and boron flowrates for forming the epi depositedfilm by epitaxial growth are determined (step S7). For example, point 34on the line 33 and a point 38 on the line 37 satisfying condition 1 andcondition 2 below are set.

Condition 1: A nitrogen concentration d1 of the epi deposited film atpoint 34 and a boron concentration d2 of the epi deposited film at point38 are equal to or higher than an impurity concentration not affected byimpurities inside the epitaxial growth equipment. In particular, thenitrogen concentration d1 of the epi deposited film at point 34 and theboron concentration d2 of the epi deposited film at point 38 are1×10¹⁵/cm³ or higher.

Condition 2: The difference (d1−d2) of the nitrogen concentration d1 ofthe epi deposited film at point 34 and the boron concentration d2 of theepi deposited film at point 38 is within a range from 1×10¹⁴/cm³ to1×10¹⁵/cm³. A flowrate n of nitrogen at the time of epitaxial growth isdetermined by the determined point 34 and a flowrate b of boron at thetime of epitaxial growth is determined by the determined point 38.

Next, a low-concentration epi deposited film is formed by epitaxialgrowth (step S8). For example, with the temperature of a silicon carbidesemiconductor substrate from 1580 to 1725 degrees C., using H₂ gas acarrier gas, SiH₄ gas and C₃H₈ gas are supplied concurrently to thesurface of the silicon carbide semiconductor substrate, and a dopant gasincluding nitrogen and boron is supplied, forming an epitaxial layer.The nitrogen flowrate in the dopant gas is set as the flowrate ndetermined at step S7 and the boron flowrate in the dopant gas is set asthe flowrate b determined at step S7.

Thus, a series of operations according to the flowchart ends. Byimplementing the present flowchart, the n⁻-type epitaxial layer 2 havingan impurity concentration from 1×10¹⁴/cm³ to 1×10¹⁵/cm³ is formed byepitaxial growth on the n-type silicon carbide substrate 1.

In the embodiment, although a condition-setting and formation session isperformed with respect to nitrogen and boron two times each, thecondition-setting and formation session may be performed three times ormore. In this case, the relationship of the added nitrogen flowrate andthe concentration of nitrogen in the epi deposited film, and therelationship of the added boron flowrate and the concentration of boronin the epi deposited film may be obtained by calculating a straight linenearest plotted measurement points.

In an EXAMPLE, on an Si face of an n-type single crystal 4H—SiC (4-layerperiodic hexagonal silicon carbide) substrate having a thickness of 365μm and a diameter of 100 mm, an n⁻-type epitaxial layer having a setconcentration of 4×10¹⁴/cm³ and a set thickness of 100 μm was formed byepitaxial growth.

The epitaxial growth of the n⁻-type epitaxial layer on the n-type singlecrystal 4H—SiC substrate was performed in a quartz reaction tube havinginstalled therein, a SiC susceptor covered by an insulating material.Here, first-half grinding, first-half polishing, and chemical mechanicalpolishing (CMP) processes were performed for the Si face that is an epigrowth surface, pure water ultrasonic cleaning/organic solventultrasonic cleaning/Sulfuric Acid Peroxide Mixture (SPM) cleaning/RCA(wet cleaning using strong acid and high basic solution) cleaning wereperformed, and the n-type single crystal 4H—SiC substrate in asufficiently clean state was set on a susceptor in a chamber ofepitaxial growth equipment. Next, after the inside of the epitaxialgrowth equipment was high-vacuum exhausted, H₂ gas was introduced intothe epitaxial growth equipment, and the single crystal 4H—SiC substratewas heated by high-frequency induction heating.

The H₂ gas, which is a carrier gas, was flowed at 150 slm for 18 minutesat 1605 degrees C. and the n-type single crystal 4H—SiC substrate wasetched.

The temperature was increased to an epi growth temperature of 1630degrees C. At 1660 degrees C., the first condition-setting and formationsession was performed flowing H₂ gas at 100 slm, flowing SiH₄(monosilane) gas at 185 sccm, flowing C₃H₈ gas at 74 sccm, flowing a gashaving a nitrogen concentration of 10% diluted by hydrogen at 30 sccm,and setting a pressure of 10300 Pa, and a single crystal 4H—SiC epideposited film was formed by epi growth. In the single crystal 4H—SiCepi deposited film, the concentration of nitrogen, which was the dopant,was 3.4×10¹⁵/cm³.

By a same technique, a gas having a nitrogen concentration of 10%diluted by hydrogen was flowed at 60 sccm and the secondcondition-setting and formation session was performed, forming a singlecrystal 4H—SiC epi deposited film by epi growth. In the single crystal4H—SiC epi deposited film, the concentration of nitrogen, which was thedopant, was 7.5×10¹⁵/cm³.

By a same technique, a gas having a boron concentration of 10% dilutedby hydrogen was flowed at 50 sccm and the third condition-setting andformation session was performed, forming a single crystal 4H—SiC epideposited film. In the single crystal 4H—SiC epi deposited film, theconcentration of boron, which was the dopant, was 3.8×10¹⁵/cm³. By asimilar technique, a gas having a boron concentration of 10% diluted byhydrogen was flowed at 90 sccm and the fourth condition-setting andformation session was performed, forming a single crystal 4H—SiC epideposited film. In the single crystal 4H—SiC epi deposited film, theconcentration of boron, which was the dopant, was 7.3×10¹⁵/cm³.

From the results of the first condition-setting and formation session tothe fourth condition-setting and formation session, as a condition forforming a low-concentration epi deposited film by epitaxial growth, agas having a nitrogen concentration of 10% diluted by hydrogen is to beflowed at 53 sccm, and a gas that has a boron concentration of 10%diluted by hydrogen is to be flowed at 77 sccm were determined.

From the first condition-setting and formation session to the fourthcondition-setting and formation session, similarly, the n-type singlecrystal 4H—SiC substrate was etched and thereafter, at 1660 degrees C.,H₂ gas was flowed at 100 slm, SiH₄ gas was flowed at 185 sccm, C₃H₈ gaswas flowed at 74 sccm, a gas having a nitrogen concentration of 10%diluted by hydrogen was flowed at 53 sccm, and a gas having a boronconcentration of 10% diluted by hydrogen was flowed at 77 sccm and witha pressure of 10300 Pa, a single crystal 4H—SiC epi deposited film of100 μm was formed by epitaxial growth. The impurity concentration of thesingle crystal 4H—SiC epi deposited film formed by epitaxial growth was4×10¹⁴/cm³. Values in the EXAMPLE are one example and variousmodifications are possible.

As described, according to the silicon carbide semiconductor substrateof the embodiment, by making the concentration of nitrogen in the epideposited film 1×10¹⁴/cm³ to 1×10¹⁵/cm³ higher than the concentration ofboron in the epi deposited film, a low-impurity-concentration epideposited film having an impurity concentration from 1×10¹⁴/cm³ to1×10¹⁵/cm³ may be formed. Further, by making the concentration ofnitrogen in the n⁻-type epitaxial layer and the concentration of boronin the n⁻-type epitaxial layer equal to or higher than a concentrationthat is not affected by impurities inside the epitaxial growthequipment, the effects of the impurities inside the epitaxial growthequipment may be reduced.

Further, impurities inside the epitaxial growth equipment are nitrogenand boron and therefore, nitrogen and boron are respectively set as adonor and acceptor whereby effects of impurities inside the epitaxialgrowth equipment may be reduced. Further, since the difference of theconcentration of nitrogen in the epi deposited film and theconcentration of boron in the epi deposited film suffices to be from1×10¹⁴/cm³ to 1×10¹⁵/cm³, the concentration of nitrogen in the epideposited film and the concentration of boron in the epi deposited filmmay be adjusted thereby enabling carrier lifetime to be controlled.

Even when C/Si increases and the atmosphere in the furnace varies, bothnitrogen and boron are atoms entering the carbon side. As a result, evenwhen C/Si increases, the relationship of the concentration of nitrogenin the dopant gas and the concentration of nitrogen in the epi depositedfilm, and the relationship of the concentration of boron in the dopantgas and the concentration of boron in the epi deposited film varysimilarly. Therefore, taking the difference becomes the same and a C/Siincrease does not affect the nitrogen concentration of the epi depositedfilm or the boron concentration of the epi deposited film.

Further, in the present invention, on a silicon carbide semiconductorsubstrate, before a low impurity concentration epi deposited film isformed, a condition-setting and formation session is performed fourtimes. Although it takes time to perform the four condition-setting andformation sessions, without the four condition-setting and formationsessions, formation of a low impurity concentration epi deposited filmfails and compared to repeating formation processes, less time isconsumed.

However, with the conventional techniques, concentration control withrespect to depth in the epi deposited film is difficult and at shallowareas, the impurity concentration increases to 1×10¹⁵/cm³ or higher,making formation of a low-concentration epi deposited film of less than1×10¹⁵/cm³ impossible.

This results because the impurity concentration of the epi depositedfilm is low and consequently, the effects of impurities inside theepitaxial growth equipment for forming the single crystal SiC enteringthe epi deposited film cannot be ignored and an epi deposited filmhaving an impurity concentration higher than a target impurityconcentration is formed.

An epi deposited film having an impurity concentration higher than atarget impurity concentration is formed as described in more detailherein. FIG. 4 is a cross-sectional view of a structure of epitaxialgrowth equipment 4. In the epitaxial growth equipment 4, in a directionindicated by an arrow, carrier gas and dopant gas are introduced andmove from a gas inlet 43 to a gas outlet 44 in a horizontal direction.The dopant gas is mainly supplied to a surface side of a semiconductorsubstrate 42, a susceptor 41 is heated by non-depicted heating equipmentsuch as an induction heating (IH) coil, and an epi deposited film isformed on the semiconductor substrate 42. The epitaxial growth equipment4 in FIG. 4 is an example of a horizontal hot wall design and althoughthe susceptor 41 rotates, the semiconductor substrate 42 does not andthe epi deposited film is formed without the semiconductor substrate 42rotating. Further, the epitaxial growth equipment 4 is surrounded by aninsulating material 45 that includes a graphite member.

With atmospheric exposure during maintenance, nitrogen is taken in bythe graphite member of the epitaxial growth equipment 4. Therefore,before formation of the epi deposited film is performed, to determineconcentration dependency with respect to nitrogen, conditions are setand formation is performed. Based on these results, actual formation ofthe epi deposited film is performed.

FIG. 5 is graph of results obtained when conditions were set andformation was performed to determine concentration dependency in aconventional silicon carbide semiconductor substrate. In FIG. 5, thevertical axis indicates nitrogen concentration of an epi deposited filmformed on a silicon carbide semiconductor substrate. The nitrogenconcentration is indicated in units of /cm³. The horizontal axisindicates a flowrate of nitrogen in the dopant gas. The flowrate isindicated in units of standard cubic centimeters per minute (sccm) orstandard liters per minute (slm).

First, as first session of condition-setting and formation session, adopant gas including a predetermined concentration of nitrogen issupplied and on a silicon carbide semiconductor substrate, for example,an epi deposited film of 10 μm is formed and the concentration ofnitrogen in the formed epi deposited film is measured. This result isrepresented by point 51. Next, as a second session of condition-settingand formation session, the dopant gas including nitrogen at a flowratedifferent from that in the first session of condition-setting andformation session is supplied and on a silicon carbide semiconductorsubstrate, for example, an epi deposited film of 6 μm is formed and theconcentration of nitrogen in the formed epi deposited film is measured.This result is represented by point 52.

Next, a line 53 connecting point 51 and point 52 is calculated. The line53 represents the relationship of the nitrogen flowrate in the dopantgas and the concentration of nitrogen in the epi deposited film. Fromthis line, it is found that when an epi deposited film having aconcentration dt of point 54 is formed, the nitrogen flowrate in thedopant gas is to be set to be n2.

However, when an epi deposited film of, for example, 30 μm is actuallyformed on a silicon carbide semiconductor substrate, the nitrogenconcentration of the formed epi deposited film is a position of point55, an epi deposited film having an impurity concentration higher thanthe target impurity concentration is formed, not a low-concentration epideposited film.

This results because the graphite member of the epitaxial growthequipment includes boron (B). Therefore, in the condition-setting andformation session, boron is taken into the epi film whereby theconcentration control is underestimated. Furthermore, in the actual epideposited film, consequent to exhaustion of boron from the graphitemember, exhaustion of nitrogen from the graphite member, and attachmentof SiC products on furnace walls of the epitaxial growth equipment, arate of (C/Si) carbon to silicon increases and the atmosphere inside thefurnace varies. Therefore, concentration control is underestimated. Forthese reasons, the epi deposited film is formed having an impurityconcentration that is higher than the target impurity concentration andno low-concentration epi deposited film is formed.

Thus, when the impurity concentration of the epi deposited film is low,the effects of nitrogen or boron inside the epitaxial growth equipmententering the epi deposited film cannot be ignored and an epi depositedfilm having an impurity concentration higher than the target impurityconcentration is formed. Therefore, formation of an epi deposited filmhaving an impurity concentration of 1×10¹⁴/cm³ to 1×10¹⁵/cm³ isdifficult.

As described, according to the silicon carbide semiconductor substrateof the present invention, by making the donor concentration of theepitaxial layer 1×10¹⁴/cm³ to 1×10¹⁵/cm³ higher than the acceptorconcentration of the epitaxial layer, a low-impurity-concentration epideposited film having an impurity concentration from 1×10¹⁴/cm³ to1×10¹⁵/cm³ may be formed. Further, by making the donor concentration ofthe epitaxial layer and the acceptor concentration of the epitaxiallayer equal to or higher than a concentration that is not affected byimpurities inside the epitaxial growth equipment, the effects of theimpurities inside the epitaxial growth equipment may be reduced.

The silicon carbide semiconductor substrate and the method ofmanufacturing a silicon carbide semiconductor substrate according to thepresent invention achieve an effect in that a silicon carbidesemiconductor substrate having a low-concentration epi deposited film ofan impurity concentration of 1×10¹⁴/cm³ to 1×10¹⁵/cm³ may be provided.

As described, the silicon carbide semiconductor substrate and the methodof manufacturing a silicon carbide semiconductor substrate according tothe present invention are useful for semiconductor substrates ofhigh-voltage semiconductor devices used in power converting equipmentand power supply devices such as in industrial machines.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A silicon carbide semiconductor substrate,comprising: an n-type epitaxial layer having a donor with a donorconcentration and an acceptor with an acceptor concentration, the donorconcentration and the acceptor concentration each being equal to orhigher than a concentration unaffected by an impurity inside epitaxialgrowth equipment, wherein the donor concentration is greater than theacceptor concentration so that a difference of the donor concentrationand the acceptor concentration ranges from 1×10¹⁴/cm³ to 1×10¹⁵/cm³, andthe donor is nitrogen and the acceptor is boron.
 2. The silicon carbidesemiconductor substrate according to claim 1, wherein the concentrationunaffected by the impurity is 1×10¹⁵/cm³ or higher.